| Multiphase flow is typically involved in many industrial processes,i.e.gas/oil/water three-phases flow in the petroleum transportation process,high moisture particle drying process in the fluidized bed in pharmaceutical industry.It is necessary to online monitor the above-mentioned multiphase flow for process operation and improve the process efficiency and safety.For example,the droplets in oil/gas transportation pipelines will damage the pipeline in the transportation of multiphase flow.In fluidized bed drying process,maintain a stable flow regime is the primary objective to ensure high process efficiency and safety.During fluidized bed drying,agglomeration caused by high moisture content particles will damage the heat and mass transfer and result into low product quality.It is difficult to identify the flow conditions and flow regimes based on visual observation.The system is regarded as a "black box".Therefore,it is necessary to online monitor and control the multiphase flow.Currently,two approaches,i.e.single-point based measurement and process tomography,are typically used in multiphase flow measurement.Single-point based measurement monitors the local flow information by using different probe,likes pressure transducer and optical fiber probe.The information provided by those probes are normally limited and difficult to identify the whole process.Process tomography(PT)is non-intrusive and it can provide the whole cross-sectional information.Electrical capacitance tomography(ECT),as one type of PT,has been widely used in lab-scale and industrial-scale process monitoring.However,a single tomography method cannot meet the requirement for multi-phase distribution measurement,especially water-containing multi-phase distribution.For example,ECT cannot work when measuring multi-phase distribution with high moisture content.Therefore,it is necessary to adopt other tomography technologies which can measurement high moisture content media.Microwave tomography(MWT)can obtain the complex permittivity(permittivity and conductivity),which has been proven to be used to measure high moisture content particles.Therefore,it is a good option to combine ECT and MWT to investigate complex multi-phase flow distribution.Based on COMSOL and CFD simulation,both ECT and MWT are used to conduct static and dynamic test for typical multiphase distribution.The static distribution of gas-liquid/liquid-liquid and gas-solid flow with different moisture contents were simulated and experimentally studied.The updated sensitive field based the original image was used to optimize the ECT and MWT images.Fluidized bed drying process with high moisture content particles are investigated by ECT and MWT.Drying process was investigated by CFD simulation and compared with ECT/MWT images.The main researches are as follows.(1)Circular ECT sensors with 8-electrode and MWT sensor with 16 antennas were designed and used to measure the oil-water/oil-gas multiphase static distribution.The results indicated that the ECT system is suitable for measuring the gas-liquid distribution but not suitable for the liquid-liquid distribution.The simulation results are similar to the ECT experimental results.The MWT system is not suitable for measuring the oil-gas two-phase flow distribution,but suitable for measuring the oil-water two-phase flow distribution.(2)Based on the 12-electrode circular/square ECT sensor and the 16-annantenas circular/square MWT sensor,ECT/MWT was used to measure particles with high moisture content.The results show that there are upper limits for the moisture content of the high-calibration medium and the measured medium for ECT,and the upper limits of different flow patterns are not completely consistent.When the moisture content of the high calibration medium or the measured medium is too high,the ECT system cannot work and the image is completely deformed.The simulation results are similar to the experimental results.Alternatively,the moisture content of the high-calibration mcdium and the moisture content of the measured medium are not limited for MWT.Under the fixed calibration background,the MWT imaging area of the medium with higher water content is more obvious.Excessively high moisture content particles will cause the flow pattern unable to be recognized normally,such as annular flow.For particles with moisture content of 0-30%,the lower the moisture content,the higher the quality of the ECT image when used as a high-calibration medium,and the stronger the MWT flow pattern recognition ability of the particles with a higher moisture content as the high-calibration medium.(3)The method of sensitive field updating on ECT and MWT is introduced in this paper and the influence of different capacitance normalization methods on the imaging results is analyzed.The results indicated that after updating the sensitive field with the initial ECT image with the smallest image error,the best normalization method of the reconstructed ECT image changes.Moreover,after updating the sensitive field with the initial ECT image with the largest image error,the reconstructed ECT image has the high quality.By using the MWT image to update the sensitive field for ECT image reconstruction,the error of the reconstructed ECT image is significantly smaller than that of the MWT image,and MWT image optimization can be realized.(4)Based on ECT and MWT,the drying process of high-moisture gas-solid particles was experimentally studied in a conical fluidized bed,and the influence of different inlet fluidization velocities and fluidization temperatures on the drying process were analyzed.The experimental results show that the ECT and MWT system can be used complementarily to monitor the complex high-moisture gas-solid dynamic process.When the ECT system is carrying out the experimental measurement of particles with high moisture content,the ECT system cannot complete normal imaging with high moisture content,especially when the moisture content of the particles is higher than 20%in the initial stage of the measurement.However,as the moisture content of the dried particles decreases,the ECT image returns to normal,and flow pattern recognition can be realized.The ECT system is used to identify the flow pattern,and the MWT system is used to identify the moisture change of the particles.Electrical information combined with online pressure,humidity,temperature and other single-point based measurement is a good option to monitor high moisture content particle drying process.(5)CFD simulation is carried out to investigate the fluidized bed drying process and compared with tomography measurement results.The effects of different material heights and different fluidization velocities on the fluidization of gas-solid flow are investigated.The solids concentration and velocity distribution of the particles are given and analyzed at different heights in the fluidized bed.The results show that CFD results are consistent with the ECT cold-state experimental results.The particles mainly present an annular flow in the fluidized bed with a core-shaped bottom;choosing the appropriate fluidization velocity and material height will help improve the accuracy of ECT imaging.The spatial resolution of ECT in the central area of the fluidized bed is low,and the particles in this area are not easily captured during fluidization.CFD numerical calculation results can provide supplementary information for the "soft field effect" of ECT.The above results indicated that ECT and MWT can complementarily applied for multiphase flow monitoring in industrial process,such as oil-water/oil-gas two-phase flow pattern recognition and high-moisture particle drying process monitoring.The updating sensitive field can optimize the reconstructed images both for ECT and MWT.MWT can make up the shortcomings of ECT to measure high moisture content materials. |
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